Hydraulic hybrid vehicle method of safe operation

ABSTRACT

A hydraulic hybrid vehicle includes elements such as a hydraulic pump driven by an internal combustion engine and arranged to draw in low pressure fluid and pump the fluid at high pressure to an accumulator. A hydraulic motor is powered by the pressurized fluid. Safety processes are provided for detecting and addressing a number of conditions that may arise in the operation of the hydraulic hybrid vehicle, including an initialization procedure for start-up of the vehicle, a shut-down procedure, and procedures for detecting and responding to failure of the pump or motor, internal and external fluid leaks, and non-responsive actuation and mode control systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure is directed to processes for safe operation of ahydraulic hybrid vehicle system, and in particular, to processes fordetecting and/or addressing safety conditions arising out of operationof the vehicle.

2. Description of the Related Art

Significant interest has been generated, in recent years, in hybridvehicle technology as a way to improve fuel economy and reduce theenvironmental impact of the large number of vehicles in operation. Theterm hybrid is used in reference to vehicles employing two or more powersources to provide motive energy to the vehicle. For example, electrichybrid vehicles are currently available that employ an internalcombustion engine and a generator which generates electricity that canbe stored in a battery of storage cells. This stored energy is thenused, as necessary, to drive an electric motor coupled to the drivetrain of the vehicle.

Hybrid vehicles may be grouped into two general classes, namely,parallel hybrid and series hybrid vehicles. Parallel hybrid vehicles arevehicles employing a more or less typical engine, transmission, anddrive train, with additional components providing a second power pathfor the vehicle. According to one parallel hybrid scheme, the engine ofa vehicle is used to generate surplus energy during periods when thevehicle is cruising at a steady speed, or otherwise demanding less thanthe engine is capable of providing when operating at its most efficientload. The surplus energy is then stored for future use.

It is known that internal combustion engines used in conventional motorvehicles are required to have a maximum output capacity that far exceedsthe average requirements of the vehicle, inasmuch as such vehiclesoccasionally demand power output levels far exceeding the average poweroutput, such as during acceleration from a stop, or for passing, etc.During these relatively brief periods of operation, much more power isrequired than during periods when the vehicle is cruising at a steadyspeed. Because of this requirement for a high level of available power,the engines in most conventional vehicles spend most of their timeoperating well below their most efficient speed and load.

By using excess capacity of the engine to produce energy that can bestored, the load on the engine can be increased to a point where theengine operates at a high level of fuel efficiency when in operation,while the excess energy is stored. The stored energy may then be used toenable engine-off operation, or to supplement the engine during periodswhen power requirements of the vehicle exceed the engine's maximumefficient output. Hybrid electric vehicles that are currently availablegenerally operate according to the scheme broadly outlined above,utilizing a generator to add load to the engine and convert the excesspower to electricity for storage in the battery, and later utilizing thebattery and an electric motor to supplement the conventional drivetrainwhen more power to the wheels is required than can be efficientlyproduced by the engine alone.

There are other parallel hybrid vehicle configurations that have beenproposed, that refine the basic system outlined above, or that providesome improved economy without departing significantly from the moreconventional model. These other systems will not be discussed in detailhere.

Series hybrid vehicles, in contrast to the parallel hybrid model, haveno direct mechanical drivetrain between the engine and the drive wheelsof the vehicle. They do not employ a drive shaft as described withreference to parallel hybrid vehicles. In a series hybrid vehicle, powerfrom an engine is converted directly to a form that can be used by asecondary drive motor to power the vehicle, and that is also conduciveto efficient storage. The engine can be operated at its most efficientload and speed without regard to variations in the speed of the vehicle.Depending on the capacity of the energy storage medium, a series hybridvehicle may operate for extended periods with the engine shut down,operating on stored energy alone. Series hybrid vehicles are potentiallymore efficient than parallel hybrids because of the greater freedom tocontrol engine operation for maximum efficiency, and because of theelimination of the mechanical drivetrain linking the engine to thewheels, thereby reducing the net weight of the vehicle, as compared to aparallel hybrid vehicle.

While electric hybrid vehicles have been briefly mentioned above, thereis growing interest in the development of hydraulic hybrid vehicles, dueto the potential for greater fuel economy, lower operating costs, and alower environmental impact, as compared to electric hybrid vehicles. Thegreater fuel economy arises in part because of the relative superiorefficiency of hydraulic systems in converting kinetic energy to astorable form, and in reconverting the stored energy potential tokinetic energy. The potential for lower operating costs is due to thefact that electric storage batteries currently available for use inhybrid vehicle operation are expensive and have a limited lifespan, withpotential replacement at significant cost to the vehicle owner. Thestorage batteries are also an environmental concern because they containlarge amounts of heavy metals that must be disposed of when the worn-outbatteries are removed from the vehicles. Hydraulic systems such as mightbe employed in hybrid vehicles do not employ components that inherentlyrequire replacement, nor do they employ large amounts of toxic orharmful substances.

The configuration and operation of parallel and series hybrid vehiclesare described in greater detail in the following references: U.S. Pat.No. 5,887,674, U.S. patent application Ser. No. 09/479,844, and U.S.patent application Ser. No. 10/386,029, all of which are incorporatedherein by reference, in their entirety.

BRIEF SUMMARY OF THE INVENTION

According to various embodiments of the invention, safety processes areprovided for detecting and addressing a number of conditions that do ormight arise in the operation of a hydraulic hybrid vehicle system.

The disclosed embodiments include an initialization procedure forstart-up of a hydraulic hybrid vehicle, as well as a shut-downprocedure. Additionally, procedures for detecting and responding tofailure of a motor, internal and external fluid leaks, andnon-responsive actuation and mode control systems are provided.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic diagram of a hydraulic hybrid vehicle systemaccording to an embodiment of the invention.

Each of FIGS. 2-7 is a flow diagram illustrating a process related tosafe operation of a hydraulic system such as that illustrated in FIG. 1,according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed to innovations and improvements inhybrid hydraulic technology. Accordingly, where reference is made tohybrid vehicles, or hybrid technology, it may be assumed that thereference is directed to hydraulic hybrid vehicles, in particular,unless otherwise noted. Aspects of the invention will be described withreference to a bent-axis pump/motor, such as is well known in the art,though, where a claim recites a motor, the scope of the claim includesany hydraulic machine that can be adapted to operate as claimed, and mayinclude, for example, swash plate and radial piston machines.

FIG. 1 is a simplified schematic diagram of a hydraulic hybrid vehiclesystem 100. The vehicle 100 includes an internal combustion engine (ICE)102 whose output shaft is coupled to a hydraulic pump 104. The pump 104is configured to draw low-pressure fluid from a low-pressure accumulator106 (LPA) and pump the fluid at high pressure to a high-pressureaccumulator 108 (HPA). The high-pressure fluid is used to drive one ormore hydraulic pump-motor(s) (hereafter motor) 110, which in turnapplies torque to drive wheels 112 via axles 114 and a differential (notshown). According to an alternate embodiment, a transmission is alsoprovided.

The pump 104 is shown as a fixed-displacement pump, but it may be avariable displacement pump, in which case the load on the engine can bemodified by changing the displacement of the pump. Additionally, thepump 104 may be a pump/motor to permit the use of the pump 104 as amotor to start the engine by fluid pressure.

The motor 110 of the embodiment described is a positive-anglepump/motor, i.e., capable of stroking from zero to a positive strokeangle. The motor may alternatively be an over-center pump/motor, capableof stroking to both positive and negative stroke angles. Thus, where,for example, the specification or claims refer to a stroke angle ofgreater than zero, this is to be construed as meaning an absolute valuegreater than zero, i.e., either in a negative or positive direction. Itwill also be recognized that the fluid circuit will be arrangeddifferently than shown in FIG. 1 to accommodate an over-center motor.Nevertheless, it is within the abilities of one of ordinary skill in theart to adapt the embodiments described hereafter for use with anover-center motor. U.S. patent application filed on Sep. 29, 2006, andbearing attorney docket number 310121.434 provides details of anover-center fluid circuit. Other circuits are known in the art.

The LPA and HPA are pre-charged with gas cells 107, 109, respectively,that are compressed as fluid is pumped into the respective accumulator.The pressure of the compressed gas provides the motive force for thehydraulic operation of the vehicle system 100.

A mode valve 116 is provided to control polarity of the fluid to themotor 110, and an actuator 118 is coupled to the motor 110 to controldisplacement. A control valve 120 controls operation of the actuator118. Low-pressure fluid lines 124 couple the LPA 106 to the pump 104 andthe valves 116, 120, while high-pressure fluid lines 126 couple the HPA108 to the pump 104 and the valves 116, 120. Motor fluid supply lines128 couple the mode valve 116 to the motor 110, and actuator fluidsupply lines 130 couple the control valve 120 to the actuator 118.

The mode valve 116 is shown as a three-position valve. In a firstposition, the valve 116 places a first fluid port of the motor 110 influid communication with the high pressure fluid supply while placing asecond fluid port of the motor 110 in fluid communication with thelow-pressure fluid supply. In this configuration, the motor 110 appliesa forward torque to the drive wheels 112, tending to drive the vehicleforward. In a second position, the valve 116 places the first fluid portof the motor 110 in fluid communication with the low-pressure fluidsupply 106 while placing the second fluid port of the motor 110 in fluidcommunication with the high-pressure fluid supply 108. In thisconfiguration, the motor 110 applies a reverse torque to the drivewheels 112, tending to drive the vehicle 100 in reverse. In a thirdposition, the valve 116 places the first fluid port in fluidcommunication with the second fluid port, creating a closed loop, inwhich condition the motor 110 is free to rotate, but does not receiveany motive power.

The control valve 120 is also shown as a three position valve, and theactuator is shown as a double action (push-pull) actuator. In a firstposition, the control valve 120 places a first fluid chamber of theactuator 118 in fluid communication with the HPA 108 while placing asecond fluid chamber of the actuator 118 in fluid communication with theLPA 106. In this position, the motor 110 is stroked toward a maximumdisplacement, which will increase the power output of the motor 110. Ina second position, the control valve 120 places the first fluid chamberof the actuator 118 in fluid communication with the LPA 106 whileplacing the second fluid chamber of the actuator 118 in fluidcommunication with the HPA 108. In this position, the motor 110 isstroked toward a minimum displacement, decreasing the power output ofthe motor 110. In a third position, the control valve 120 closes theactuator supply lines 130, hydraulically locking the actuator 118 inplace and holding the displacement of the motor 110 at a constant value.

Pilot controlled check valves 138, 140 are positioned in the high- andlow-pressure fluid lines 126, 124, respectively. Valves 138, 140 arearranged to always permit fluid flow into the respective accumulator toavoid the possibility of excess pressure build up in the respectivefluid lines. When one or the other of these valves are opened, asmentioned in several of the disclosed embodiments, this refers to thepilot operation of the valve, in which fluid flow away from therespective accumulator is also enabled. Where the claims recite couplinga high- or low-pressure fluid supply to a system, this is to beconstrued to include opening a one-way valve to two-way operation, suchas described with reference to valves 138, 140.

A number of sensors are provided to monitor various aspects of theoperation of the vehicle 100. These include sensors 134, 136 coupled tothe high- and low-pressure accumulators 108, 106, respectively,configured to measure fluid pressure and flow rate; rotation sensors 146(only one of which is shown) for measuring rotation speed of the wheels112; position sensor 156 for determining the stroke angle of the motor110; pressure sensor 144 for measuring a fluid pressure inside thecasing of the motor 110, and another (not shown) for measuring a fluidpressure inside the casing of the pump; flow sensor 142 positioned inone of the motor fluid supply lines 128; shift indicator sensor (PRNDL)148 for detecting a position of a shift indicator; accelerator positionsensor (APS) 150 for monitoring a position of an accelerator pedal;brake position sensor (BPS) 152 for monitoring a position of a brakepedal; and key position sensor (KPS) 154. Additionally, the ICE 102 isprovided with a typical suite of engine sensors (not shown separately)such as are commonly employed in modern vehicles, such as, for example,throttle position sensor, coolant temperature sensor, oil pressuresensor, rpm sensor, etc.

Either or both of the accumulators 106, 108 may include a “fuse” valve(not shown) configured to close if a maximum flow rate value isexceeded. Any or all of the check valves 138, 140, the flow rate sensors134, 136, and the fuse valves may be integrated, and may be located inor on the respective accumulators 108, 106.

The position sensor 156 is shown coupled to the actuator 118, though theposition sensor 156 may instead be positioned at or in the motor 110.According to an embodiment, there may be more than one position sensor156, as will be explained later in this disclosure. The shift indicatorsensor 148 detects a position of a shift indicator via which a driverselects a mode of vehicle operation from among a number of choices suchas, for example, P (park), R (reverse), N (neutral), D (drive), and L(low, in those embodiments that include a transmission and/or one ormore additional motors).

A control unit 122 controls many aspects of the operation of the vehicle100, including the ICE 102, the mode valve 116, the control valve 120,and the check valves 138, 140. Control lines 132, shown in dotted lines,are coupled between the control unit 122 and components of the system,including the mode and actuator control valves 116, 120, the checkvalves 138, 140, and the ICE 102. Control lines 132 also couple thecontrol unit to the various sensors of the system. The check valves 138,140 and valves 116, 120 may be controlled by any of electrical,mechanical, or hydraulic means, such as is well known in the art.Additionally, the sensors may provide data to the control unit by any ofa number of mediums, including digital or analog electrical signals,hydraulic or pneumatic pressure, mechanical linkage, etc. Accordingly,the control lines 132 are shown merely to indicate an operativeconnection, and are not intended to suggest the nature of theconnection.

The ICE 102 drives the pump 104, which pumps fluid at high-pressure intothe HPA 108. The control unit 122 monitors the fluid pressure in the HPAand controls the operation of the ICE 102 to maintain the fluid pressurewithin an acceptable range. Pressurized fluid from the HPA 108 isutilized to control the displacement of the motor 110 and to power themotor 110 to apply torque to the drive wheels 112. A number of schemesfor managing the operation of an ICE in a hydraulic hybrid vehicle havebeen proposed, any of which may be appropriately employed with theembodiment described here. Some of these schemes may be found in thepreviously listed U.S. patents and U.S. patent applications.

The vehicle 100 is also configured to employ regenerative braking. Whenthe vehicle is traveling forward, and the operator applies the brake,the mode valve 116 is moved to its second position, which, as describedabove, causes the motor 110 to apply torque in the reverse direction.The amount of reverse torque is controlled by the displacement of themotor 110, which in turn is controlled by the amount of pressure (ordepression) applied by the operator on the brake pedal. Because of theforward motion of the vehicle 100, the motor 110 continues rotation inthe forward direction, even though it is applying torque in the reversedirection. In this configuration the motor 110 operates as a pump,drawing fluid from the LPA 106 and pumping the fluid at high pressure tothe HPA 108. This creates drag on the rotation of the axles 114, whichis transferred to the drive wheels 112, slowing the vehicle 100. In thisway, a portion of the kinetic energy of the moving vehicle is recoveredand stored for later use. Thus, energy that would otherwise be lost tofriction in the brakes of the vehicle is recovered and stored, to bereleased later, e.g., to assist the vehicle 100 in accelerating.

To operate the vehicle 100, a driver turns a key or otherwise selectsstart-up of the vehicle 100. The control unit 122 is powered up, the KPSdetects the start-up command, and the control unit initializes thesystem. When the shift indicator indicates a forward “gear” or position,the control unit 122 controls the mode valve 116 and the control valve120 according to a position of the accelerator pedal or brake pedal.When the driver depresses the accelerator pedal, the mode valve 116 ismoved to the first position, applying fluid pressure to the motor 110,and the control valve 118 is controlled to increase displacement of themotor, converting fluid pressure to torque to accelerate the vehicle.When the driver lets up on the accelerator, the displacement of themotor is reduced accordingly, and, if the accelerator pedal is fullyreleased, the mode valve may be switched to the neutral mode, whichrelieves the motor of even the minimal drag caused by fluid pressure onthe moving parts. When the operator steps on the brake, the vehicle isslowed by regenerative braking, as described above. When the driverselects P or N at the shift indicator, the accelerator pedal isdecoupled from the operation of the motor so that the vehicle cannot bedriven in these driving modes.

In the use of hydraulic technology, some concerns arise because hybridvehicles do not share all the same operating characteristics withconventional vehicles, while other concerns are inherent to hydraulicsystems. Some potentially dangerous situations can arise through theactions of a vehicle operator, while others may be due to malfunctions.In any event, these concerns must be addressed before such vehicles canbe commercially produced or sold. It must be recognized thatconventional passenger vehicles have been evolving for over a century,and that they have become standardized to such a degree that anindividual can learn to drive in virtually any make or model of vehicle,and thereafter be fully capable of driving any other make or model ofvehicle. An obvious exception to the rule is in the matter of automaticvs. standard transmissions, but even in this case, the exception is sowell known that most drivers who have not learned to operate a standardtransmission vehicle know the difference and how to recognize such avehicle.

With the advent of hybrid vehicles, there are many previously unknownsituations that might arise, for which a typical driver may beunprepared. The inventor has recognized that to the extent a hybridvehicle can be made to interact with a driver in a manner that issubstantially similar to a conventional vehicle, the driver will bebetter equipped to react appropriately to everyday occurrences, as wellas most emergencies.

The flow diagrams of FIGS. 2-7 outline operation of a hydraulic hybridvehicle according to various embodiments of the invention. In detailingsome of those embodiments, reference will be made to componentsdisclosed with reference to the vehicle 100 of FIG. 1. It will berecognized that the system of FIG. 1 is merely exemplary, showing one ofmany possible configurations. Other systems may not include all thefeatures described with reference to FIG. 1, and will probably includefeatures not described. Functions performed in the disclosed embodimentsby particular elements may, in other embodiments, be performed bydifferent elements. For example, where the mode valve 116 is shownschematically as a single valve, in many hydraulic systems the functionsdescribed herein with reference to the one valve are performed by twopoppet valves. With regard to control elements such as sensors and thecontrol unit, descriptions of the other embodiments of the invention maynot specifically recite a particular sensor as performing a describedfunction, but one of ordinary skill in the art will recognize disclosedelements that could be employed to fulfill the function, and will alsorecognize a number of alternative configurations that could be adaptedto operate in particular applications, given the relevant designconsiderations. Finally, the functions described with reference to thecontrol unit may be performed by a single unit such as a microprocessoror the like, may be broken up among a number of elements, or may be partof the operation of an element or elements configured to perform othertasks.

Referring now to FIG. 2, a start-up procedure 200 is disclosed,according to an embodiment of the invention. In step 202, the driverturns the key or otherwise starts initialization of the vehicle'soperation system. The control unit confirms that the shift selector isin either park or neutral (204). If not, i.e., if the shift selector isin one of the forward settings or the reverse setting, the driver isnotified of the cause for a non-start (214), and no further action istaken. If the shift selector is in the Park or Neutral position, thecontrol unit confirms that the mode valve 116 is in the neutral position(206), and, if so, that the motor 110 is set at zero displacement (208).If the motor 110 is set at zero displacement, the high-pressure valve138 is opened (210), and normal operation of the vehicle is enabled(212).

If, at step 208, the motor displacement is found to be greater thanzero, the high-pressure valve is opened (228), and the motor iscommanded to a displacement of zero (230). The displacement is againchecked (232), and if the motor 110 has moved to a displacement of zero,the system is enabled for normal operation (212). If the motor 110 hasnot moved to a zero displacement, the high-pressure valve 138 is closed,and the operator is alerted to a critical system fault (CSF) condition(240), i.e., that the system is inoperative and in need of repair orservice before the vehicle can be operated.

Where a CSF condition is signaled to an operator, this may be as basicas a light or other indicator on the vehicle instrument panel, incombination with the vehicle system shutting down, or the signal mayprovide more detailed information such as the nature of the fault, thedefective component, or the type of service required.

Returning to step 206, if the mode valve is found to be at a positionother than neutral, the control unit attempts to confirm that the motordisplacement is at zero (216). If the motor displacement is not at zero,the system signals a CSF to the driver (218). If the motor displacementis confirmed to be at zero at step 216, the high-pressure valve 138 isopened (220), and the mode valve is commanded to neutral (222). Again,the position of the mode valve is checked. If it has moved to neutral,the system is enabled for normal operation (212). If the mode valve hasnot moved to neutral, the high-pressure valve 138 is closed, and theoperator is alerted to a CSF condition (226).

The time required to move through the process outlined above may be inthe range of a few hundred milliseconds to as much as a second, but,with regard to the operator's perception, it can be performed almostinstantaneously, so that the operator can “start” the vehicle and driveaway without any perceived delay.

It will be recognized that there is no absolute need to require that theshift indicator be in Park or Neutral to start up the system of thevehicle 100, inasmuch as, with the mode selector in neutral and thedisplacement at zero, there would be no power transfer to the wheelsduring start-up. However, a driver who has learned to drive in aconventional vehicle may be accustomed to applying pressure to theaccelerator pedal as the key is turned, to provide extra fuel to theengine during start-up. During normal operation of the hybrid vehicle100, pressure on the accelerator pedal is measured to establish thedisplacement of the motor 110, so if the pedal were depressed duringstart-up, the vehicle could jump forward unexpectedly as soon asinitialization was complete. Rather than attempting to modify thebehavior of the driver, operational safety is more surely obtained byestablishing the artificial requirement of shifting to P or N to start(with control of motor displacement disabled in these settings).According to an alternate embodiment, the start-up procedure outlined inFIG. 2 also includes starting the ICE when the system is enabled at step212, even if the stored energy in the HPA is sufficient to operateinitially without the ICE. This start-up of the ICE will provide thedriver with familiar cues that the vehicle is powered up and ready todrive. The control unit may also be programmed to slave the throttlecontrol of the ICE to the accelerator pedal to permit revving of the ICEduring start-up and before the shift selector is moved away from the Por N positions, for reasons similar to those previously mentioned (asused here, the term throttle, e.g., throttle control, or throttleposition, refers to the fuel rate of the ICE).

According to embodiments of the invention, there are sub-routines thatmay be performed serially or concurrently with the start-up processoutlined above as part of an initialization of a vehicle system. Forexample, FIG. 3 outlines a check process 300 for the low-pressure sideof the hydraulic circuit of the vehicle, according to one embodiment.

After the operator turns the key (202), the low-pressure valve 140 isopened (302), which permits fluid to flow from the LPA 106 into thesystem. Fluid flow from the LPA is then measured (304) and compared to athreshold value (306). If there is a fluid flow that exceeds thethreshold, the pressure valve 140 is closed and the operator is alertedto a CSF condition (308). If, on the other hand, any fluid flow is belowthe threshold, the system can proceed with opening the high-pressurevalve 138 in accordance with one of the steps outlined with reference toFIG. 2, such as for example, at step 210.

In a typical hydraulic system, some leakage is normal, as fluid escapespast valves, seals, and pistons. However, under the circumstancesoutlined with reference to FIG. 3, the system is closed, there is noenergy transfer underway, and none of the components are in operation.In a closed and inactive system, the only places that the low-pressurefluid might be expected to flow to are the casings of the pump 104 andthe motor 110. Accordingly, any flow rate detected would be expected tobe very modest. Thus, if the flow rate exceeds the threshold, this is anindication that there is a fluid leak to the outside of the system, suchas from a ruptured line, a defective fitting, etc. Even then, thecontrol unit may be programmed to warn the operator of a low-pressureleak, indicated by a flow exceeding the threshold, but otherwise permitoperation of the system, unless the flow exceeds a second threshold,indicating a more serious rupture.

FIG. 4 illustrates a similar process for checking the high-pressure sideof the fluid circuit. Following the key on step (202), the high-pressurevalve 138 is opened, such as at step 210, 220, or 228 of FIG. 2, whichpermits fluid to flow from the HPA 108 into the system. Fluid flow atthe HPA is then measured (402) and compared to a threshold value (404).If there is a fluid flow that exceeds the threshold, the high-pressurevalve 138 is closed and the operator is alerted to a CSF condition(406). If fluid flow is below the threshold, operation of the system isenabled, at least with respect to the concerns addressed in the processof FIG. 4 (408).

As with the process outlined in FIG. 3, measuring flow of high-pressurefluid into the system may detect leaks, including leaks out of thesystem. As previously explained, there is generally some internalleakage that is inherent in a hydraulic system, which is tolerable in anormally operating system. It will be recognized that high-pressurefluid flowing past valves or seals of the system to the low-pressureside of the circuit will flow into the LPA 106. If the fluid flowexceeds the threshold, this indicates either a defective component orseal in the system, which is allowing high-pressure fluid to escape tothe low-pressure side of the circuit, or it indicates a leak of fluidout of the system. Either fault is sufficient to prompt a CSF condition.

In a closed system, the fluid flow at the HPA will be equal to the flowat the LPA, and any difference in flow between the HPA and the LPAindicates a loss of fluid from the system, i.e., a leak outside thesystem. This process is outlined in FIG. 5. After the key is turned on(202) and the high-pressure valve is opened (210), the fluid flow at theLPA is measured (502). Additionally, the flow at the HPA is measured(504). The high-pressure accumulator flow (HPAF) is compared to thelow-pressure accumulator flow (LPAF) (506). If an absolute value of thedifference between the HPAF and the LPAF exceeds a threshold, the systemcloses the high-pressure valve and alerts the driver of a leak outsidethe system (508). If the difference does not exceed the threshold, thesystem is enabled for start-up (510).

Referring now to FIG. 6, another process 600 is provided for detectingexcessive external or internal leaking of a hydraulic vehicle systemsuch as that described with reference to FIG. 1, during normal operationof the vehicle. Fluid flow at the HPA is measured (602). A HPA flowvalue is calculated, based upon operating characteristics of the motor110 and/or pump 104 (604), and compared with the measured fluid flow.The difference between the calculated value and the measured valuerepresents the amount of fluid that is leaking past components in thesystem. If leakage exceeds a threshold value (606), the motor iscommanded to a zero displacement (608), the mode valve is commanded toneutral position (610), the ICE is shut down (612), the HPA valve 138 isclosed (614), the LPA valve 140 is closed (616), and the driver isalerted to a CSF condition (618). Steps 608-616, indicated by referencenumber 620, will hereafter be referred to collectively as an autoshut-down procedure.

In more detail with regard to step 604, when the motor 110 is operatingproperly, the volume of fluid passing through the motor can becalculated very accurately. For example, if the displacement of themotor 110 is set at 10 cubic inches (in³), a single rotation of themotor 110 will move 10 in³ of fluid through the motor. Thus, a fluidflow for comparison can be calculated by multiplying the knowndisplacement by the rpm of the motor. Any difference is due to leakage.The process of FIG. 6 is best performed when the motor 110 is operatingwhile the pump 104 is not in operation. This avoids the addedcomplication of factoring the displacement of the pump 104 into theequation, though if both the motor 110 and pump 104 are in operation,the calculation can still be performed. By the same token, the test canbe performed while the motor 110 is inactive but the pump 104 is inoperation, using the displacement of the pump 104 to compare with themeasured flow rate.

An excessively high internal leakage rate, such as would be detected bythe process of FIG. 6, is indicative of internal damage to a pump orvalve. Performing the auto shut-down helps prevent or limit furtherdamage to the system that might occur as a result of continuedoperation.

The auto-shutdown procedure 620 is performed in situations where apotentially dangerous situation may exist and operation of the vehiclemust be terminated to prevent danger to the occupants of the vehicle,severe damage to the system, or danger to other vehicles on the road. Inthe case of a high-pressure fluid leak outside the system, such as wouldbe detected by one or more of the processes of FIGS. 4-6, if the leak isnot detected, the high pressures in the system can very quickly turn aminor leak into a very large one. Depending on the size of the vehicle,the system may have 10-20 gallons of hydraulic fluid, which is generallysome type of oil. The hydraulic lines and valves that supply fluid fromthe accumulators 106, 108 to the motor 110 are designed to accommodate aflow of more than 100 gallons per minute (gpm) at pressures exceeding4,000 psi. If, for example, a hose fitting were to burst, the entirefluid contents of the system could be deposited on the road behind thevehicle in less than ten seconds. Such a volume of oil being pouredunexpectedly on a highway could create a hazardous situation.

On the other hand, when performing an auto shut-down, it is importantthat the order of steps be such that the vehicle is allowed to coast toa stop, so that the driver can steer off the road, and also to avoid thepossibility that the motor 110 freezes, locking the wheels into anuncontrolled skid. Referring to FIG. 1, the actuator 118 is powered byhydraulic pressure. In many embodiments, either or both of the modevalve 116 and the actuator control valve 120 are also operated byhydraulic pressure. If either the displacement actuator 118 can be movedto zero, or the mode valve 116 can be moved to neutral, the motor 110will rotate freely, without power, allowing the vehicle to coast. Bycommanding the motor 110 to zero displacement and the mode to neutralbefore the HPA valve 138 is closed, fluid pressure in the system can beused to perform the commands. Even if the auto shut-down is in responseto a malfunction in one of these subsystems, the other subsystem willbring the motor to an effectively neutral condition. If the ICE 102 isdriving the pump 104 when the auto shut-down occurs, closing the LPAvalve 140 before shutting down the ICE will deprive the pump 104 oflow-pressure fluid, and the pump 104 will cavitate. This can damage thepump 104, and will also suddenly remove the load from the ICE, which mayover-rev as a result. Shutting down the ICE prior to closing the LPAvalve will prevent this from occurring.

Similarly, in some systems, closing the HPA valve 138 before shuttingdown the ICE may cause the pump 104 to hydraulically lock, producingvery high pressure, or the motor 110 to suddenly exert a higher outputtorque, depending, in part, on the exact circuit arrangement and thepositions of other valves in the system. Either outcome could damagecomponents of the system and create a dangerous driving condition. Thiscan be avoided by shutting down the ICE prior to closing the HPA valve.

Turning now to FIG. 7, a process 700 is provided for monitoring thedisplacement control of the motor 110. At step 702, the actual motordisplacement (AMD) of the motor 110 is measured, then compared to thecommanded motor displacement (CMD) (704). If an absolute value of thedifference between AMD and CMD does not exceed a first threshold (TD)(704), the process loops back to step 702. If the difference exceeds thefirst threshold, but not a second threshold (706), the operator isnotified of a non-critical system fault (NSF) at step 716, but theprocess loops to step 702 and the system continues in operation (thisprocess up through step 706 may also apply to pump 104 if itsdisplacement is variable). If the difference exceeds the secondthreshold at step 706, the motor 110 is commanded to zero displacement(708) and a new comparison is made to determine if, following the newcommand, the difference is now below the first threshold (710) or thesecond threshold (712). If, following the new command, the difference isbelow the first threshold, the process loops to step 702. If thedifference is below the second threshold (712), the operator is notifiedof a NSF at step 716 and the process loops to step 702. If, after thenew command, the difference still exceeds the second threshold, modevalve 116 is commanded to neutral (718). At step 720, the actual modevalve position (AMVP) of the mode valve 116 is measured, then comparedto the neutral position (722). If the AMVP is neutral, the AMD iscompared to a third displacement threshold (724) and, if the AMD is lessthan the third threshold, the operator is notified of a NSF at step 716and the process loops to step 702. If the AMVP does not return toneutral or the AMD is greater than the third threshold, an auto-shutdownis performed (620) and the operator is notified of a CSF at step 726. Inone alternate embodiment, steps 706, 708, and 710 are eliminated, andmode valve 116 is commanded to neutral (718) based on an initialdetermination at step 712 that the difference exceeds the secondthreshold.

A difference between commanded and actual motor displacement can becaused by a malfunction in the actuation control valve 120, the actuator118, or the motor 110. If the displacement changes in response to acommand, but does not fully move to the commanded displacement, this mayindicate a worn yoke bearing in the motor, a sticky piston in theactuator, or some other malfunction that introduces excessive frictionto the displacement control mechanism. Such a condition should becorrected as soon as possible, but may not require an auto shut-down,provided the mismatch is not severe. Accordingly, the driver isnotified, but the system is allowed to continue in operation. However,if the motor 110 sticks in a high displacement condition and does notmove to zero when commanded, this is the equivalent of the throttlesticking open in a conventional automobile, which can be very dangerousand requires a prompt response. The problem may be as simple as a smallbit of grit that is jammed in the valve 120. By attempting to zero thedisplacement (step 708), a transitory problem may be corrected withoutresorting to an auto shut-down.

As with the CSF, a NSF signal may simply consist of a light or otherindicator on the vehicle instrument panel indicating that service isrequired, or the signal may include more information regarding thenature of the fault.

According to an alternate embodiment, the control unit is programmed torapidly pulse the valve 120 in place of or in addition to the step ofmoving the displacement to zero, at step 708. The pulsing may serve tofree contaminants from the valve or actuator and permit the system toreturn to normal operation.

According to another embodiment, two sensors 156 are provided, eachconfigured to detect the displacement position of the motor 110. If thetwo sensors disagree as to the displacement of the motor, data from eachis compared to the commanded displacement, and the sensor that disagreeswith the commanded displacement is ignored. The driver is notified of aNSF, but the system continues in otherwise normal operation. If, on theother hand, the two sensors agree with each other, but disagree with thecommanded displacement, the process outlined in FIG. 7 is followed.

According to a further embodiment, a parallel displacement control valveis provided, such that, in the event of a stuck displacementdetermination by a process such as that illustrated in FIG. 7, thecontrol valve 120 is removed from the circuit (by a shut-off valve), andthe parallel control valve is activated to supply high- and low-pressurefluid to the actuator 118. Thus, if the problem is in the control valve,the driver is notified of a NSF, while the system remains operational.

According to an embodiment of the invention, a process is provided formonitoring the condition of the gas cells 107, 109 of the LPA 106 andthe HPA 108, respectively. It will be recognized that as high-pressurefluid flows into or out of the HPA 108, the pressure within thataccumulator will change accordingly. Likewise, as low-pressure fluidflows into or out of the LPA 106, the pressure within that accumulatorwill also change accordingly. In a closed system, such as that describedwith reference to FIG. 1, if fluid flows out of one accumulator, it mustflow into the other at the same rate. This does not mean that changes ofpressure in the accumulators will be of equal values, inasmuch as theHPA 108 is precharged to a much higher pressure than the LPA 106, butthe changes of pressure can be correlated, and the pressure of oneaccumulator can be accurately predicted, given the pressure of theother.

Thus, if the measured pressure of one of the accumulators 106, 108 isnot equal to the predicted pressure, based on the correlation with thepressure of the other accumulator, a fault condition exists, indicatingeither a loss of fluid from the system or a loss of pressure. If theloss of pressure is due to gas leaking from the gas cell 109 of the HPA108, the pressure will rise at the LPA 106, and will actually be greaterthan what would be predicted, given the pressure at the HPA 108, so thatthe sum of the pressures of the HPA and LPA is actually higher thanexpected. Thus, such a gas leak can be distinguished from a fluid leakby the excessive rise in pressure of the LPA. On the other hand, if gasescapes from the gas cell 107 of the LPA 106, the escaping gas willremain in the LPA or become entrained in the fluid of the system, whichmay not result in a change of pressure, but will cause increasedcompressibility of the fluid, resulting in slower responses and reducedefficiency of the system. If the indicated leakage exceeds a firstthreshold, but is less than a second threshold, a NSF is indicated, andif the leakage exceeds the second threshold, an auto shut-down isperformed and the operator is notified of a CSF.

According to an embodiment of the invention, the control valve 120 isconfigured to move the actuator 118 to zero the displacement of themotor 110 in the event power to the valve is lost. Depending on theconfiguration of the control valve, this may be accomplished in a numberof ways. For example one or more springs may be provided that will movethe valve to the second position, unless some other force exerts anopposing force. The opposing force is provided by the control unit, viaa pilot valve, a solenoid, or other control means. Thus, if pressure tothe pilot valve, power to the solenoid, or controlling signal from thecontrol unit is lost, the springs will immediately move the controlvalve to the second position, zeroing the motor. In this way, systemfaults that cut off power to the control valve, and that might otherwisecause the motor to be locked at some positive displacement or move to ahigher displacement, will instead cause the motor to move to a zerodisplacement. The actual configuration of such a valve will depend onfactors such as the source of power for the valve, the style of valveemployed, design of the actuator mechanism controlled by the valve, andthe type of motor being controlled by the actuator. Patent applicationnumber [safe over-center app], previously cited and incorporated herein,discloses one such control valve for use with an over-center motor.

According to another embodiment, the mode valve 116 is configured toplace the motor in an unpowered condition in the event of a loss ofpower to the valve. Thus, if power is lost to the valve for any reason,power is removed from the motor, preventing uncontrolled power to themotor. This may involve a valve having a spring configured to drive thevalve to the neutral position if power is lost.

According to an embodiment of the invention, fluid pressure is monitoredwithin the motor and pump casings. In most hydraulic pumps and motors,fluid inside the casing is vented to the low-pressure side of thesystem. In this way, fluid that inevitably leaks past the pistons andseals of the machine is returned to the low-pressure fluid supply. Inthe present embodiment, in the event of a machine failure in which alarge quantity of high-pressure fluid escapes to the casing, thepressure sensor will detect a rise in pressure, and the control unitwill execute an auto shut-down, or at the least, shut off high pressureto the machine.

According to some hybrid vehicle designs, additional motors areincorporated, either alongside the first motor, or coupled to anotherpair of drive wheels. According to an embodiment of the invention, inthe event of a system failure that results in the inoperability of amotor, the inoperative motor is shut-out, while one or more remainingmotors can operate in a “limp home” mode to permit the vehicle tooperate at reduced capacity to avoid complete shut-down. Such systemfailures may include loss of displacement control, internal and externalleaks in the fluid circuit of one or another motor, and other motorfailures. The isolation of the inoperative motor can be accomplished byplacing its respective mode valve in neutral, and commanding the motorto a zero displacement. With regard to the process described withreference to FIG. 7, the steps 718 and 720 are of particular use insystems employing more than one hydraulic motor. If the mode valve canbe confirmed to be in its neutral position, the associated motor can beisolated from the system by the mode valve, without completely shuttingdown the system. In an alternative embodiment, isolation valves areprovided, configured to separately isolate each of the motors so that,regardless of the type of failure, one motor can be completely isolatedfrom the system while allowing the system to operate with the remainingmotor(s). Vehicles employing more than one motor for operation aredescribed in a number of references, including the following references:U.S. Pat. No. 6,718,080; and U.S. patent application Ser. No.10/620,726; and U.S. patent application Ser. No. 11/233,822.

According to an embodiment of the invention, the ICE 102 and motor 110are provided with over-speed protection, such that if the control unitdetects the ICE or motor rotating at an excessive rate, power is removedfrom the machine to prevent damage.

According to an embodiment of the invention, a process is provided forcontrolling the regenerative braking system. When the driver appliespressure to the brake pedal, the control unit controls the motor toapply a reverse torque to the wheels, as described above. However, asthe speed of the vehicle drops toward zero, the controller decreases thedisplacement of the motor to reduce the braking torque. At the sametime, the friction brakes always remain operative such that ifadditional braking is required, a slight increase in pressure on thebrake pedal engages the friction brakes. Thus, if the vehicle stops on ahill, for example, it is the action of the friction brakes that holdsthe vehicle in place. This avoids a problem of attempting to balancefluid pressure to hold the vehicle unmoving on an incline. Additionally,this serves to clean the brake rotors of debris and rust so they remainfully functional in the event of a loss of power to the motor, so thatthe driver always has brakes available. Additionally, if rotationsensors 146 coupled to the drive wheels 112 detect a significantdifference in rotation of the wheels, indicating that a wheel isslipping, either while accelerating or braking, displacement to themotor is momentarily reduced to allow the slipping wheel to regaintraction.

There are a number of conditions created by the operating principles ofsome hydraulic hybrid vehicles that can result in confusing or hazardoussituations for a typical vehicle operator. For example, often, thevehicle systems are programmed to shut down the ICE if there is anadequate charge in the HPA. Thus, it is possible, when the vehicle isstopped, for the driver to forget that the vehicle is actually“running,” because there is no outward indication that there is stillpower available. A driver could park the vehicle, leaving the powerengaged, then leave the vehicle. There would then be some danger thatanother party, perhaps a small child, could enter the vehicle andunintentionally set the vehicle in uncontrolled motion. If a mechanicwere to make the same mistake, then begin to disconnect a high-pressurefluid line, fluid could be discharged with a great deal of force, withthe potential to cause serious injury.

According to various embodiments, features are provided that increasesafety while the vehicle is stopped or being shut-down by the driver.For example, if the vehicle is traveling below a threshold speed ofbetween around 1-5 mph, and if the shift indicator is in a movingposition (i.e., D, L, or R), displacement of the motor 110 is controlledto a slight positive value such that when the vehicle is stopped, thedriver must apply brakes to prevent the vehicle from creeping forward(or backward, if in reverse). This will tend to remind the driver tomove the shift indicator to the P position. According to an embodiment,when the shift indicator is moved to the P or N positions, the ICE isstarted, regardless of the state of charge of the HPA. The runningengine serves to remind the driver that the vehicle is still underpower, and that the key must be moved to the “off” position. The shiftindicator must be moved to the P position before the key can be removedfrom the slot, again reminding the driver to fully shut-down thevehicle. According to an embodiment, a pressure sensor is provided inthe driver's seat. If the driver leaves the seat without turning off thekey, or placing the shift indicator in P, an alarm is sounded to alertthe driver to the omission.

Where appropriate, the term key should be construed broadly to includeany mechanism configured to enable and/or disable operation of thevehicle, including switches or buttons, remote devices, etc.

When the key is moved to the Off position, the system shuts down in amanner similar to an auto shut-down as described with reference to FIG.6. The motor is commanded to zero displacement; the mode valve iscommanded to neutral; the ICE is shut down, the HPA valve is closed; theLPA valve is closed; and finally the control unit shuts down.High-pressure remaining in the system will gradually leak past seals andpistons to the low pressure side, until the pressure in the entiresystem is about equal to the LPA. According to an embodiment, the systemincludes a pressure relief valve that is opened after the HPA and LPAvalves are closed, which vents pressure from the high-pressure side tothe low-pressure side.

In many of the processes described in the present disclosure, someparameter of the system is measured. As used in the specification andclaims, the term measure is not limited to actually obtaining a valuefor comparison or calculation. For example, the process described withreference to FIG. 7 includes measuring the actual displacement of themotor 110. While some systems may be configured to provide a truedisplacement value, there are many alternative solutions that areacceptable. In a system employing a bent-axis motor, a yoke of the motoris rotated through an arc, and displacement of the motor is commonlydescribed in terms of stroke angle of the yoke. At a stroke angle ofzero, the displacement is also zero, while as the angle increases, sotoo does the displacement, in a precisely known relationship. If thesensor 156 is a transducer configured to provide a voltage signal thatvaries directly with the angle of the yoke, the displacement of themotor can be accurately inferred from the value of the transducersignal. Furthermore, the control signal provided by the control unit tocommand the displacement of the motor may be nothing more than a voltagesignal of a value that corresponds to the commanded displacement. Thus,the steps of measuring and comparing can be performed continuously by anelectrical circuit configured to condition one or both of the voltagesignals, from the transducer and the control unit, so that, if theactual displacement is equal to the commanded displacement, the valuesof the signals are equal, then continuously comparing the values, andoutputting a fault signal if a difference between the values exceeds areference value. It can be seen that, in the arrangement described, thedisplacement volume of the motor is not measured, in a narrow sense ofthe term, nor is such a value compared with a commanded displacementvolume. Nevertheless, such a configuration would be considered toperform the steps outlined, and thus fall within the scope of theinvention.

In a similar manner, measurement of other parameters, and performance ofother processes, may be performed inferentially. The process describedwith reference to FIG. 6 includes measuring a flow rate at the HPA, andcomparing the measured rate with a flow determined by the motordisplacement and rotation. It is well known that it is much simpler andmore economical to measure pressure of a fluid than flow. It is alsoknown that a flow rate can be determined by measuring a difference inpressure at two points in a fluid transmission line, if pressure dropcharacteristics of the line are known. Accordingly, by comparing thefluid pressure at the sensor 134 with the pressure at the sensor 142while the mode valve 116 is positioned to channel high-pressure fluidpast the sensor 142, while the control valve 120 is in its thirdposition, i.e., closed, and while the pump 104 is not in operation, theflow of fluid between the sensors can be determined, and compared to theflow indicated by the displacement and rpm of the motor as outlined inFIG. 6. if the values disagree, a fault exists in the circuit.Additional pressure sensors at other points in the fluid circuit can beused to determine fluid flow rates in other branches, confirming, forexample, that all the fluid flowing from the HPA is flowing into theLPA, as described with reference to FIG. 5.

A number of processes have been disclosed, in accordance with variousembodiments, for addressing safety concerns related to the operation ofa hydraulic hybrid vehicle system. It will be noted that if all thedisclosed processes were implemented in one vehicle, there would beconsiderable redundancy. Redundant safety processes are not intended tobe mutually exclusive. While any of the disclosed processes may beimplemented individually, utilizing redundant processes generallyresults in a significant increase in the overall safety of a system.Hydraulic systems in general are typically extremely robust andreliable. Nevertheless, in a vehicle that will be expected to remain inoperation for many years, and travel thousands of miles, the possibilityof malfunctions must be acknowledged.

Where a claim recites a step that is to be performed following theperformance of a prior, conditional, step, the claim is to be construedto mean that the following step is not to be performed unless theconditions under which the prior step is to be performed are met.Additionally, where a claim recites a step that is to be performed priorto a conditional step, the claim is to be construed to mean that theconditional step is not to be performed unless the conditions underwhich the conditional step is to be performed are met after the priorstep is performed.

The term critical system fault (CSF) is used in the claims to refer to acondition under which the system to which the term is applied cannotremain in operation, or should not remain in operation, in order toavoid damage to the system or some other hazardous condition.

The term non-critical system fault (NSF) is used in the claims to referto a condition under which the system to which the term is applied canremain in operation, but may require service.

While the method and process steps recited in the claims may bepresented in an order that corresponds to an order of steps disclosed inthe specification, unless specifically indicated, the order in whichsteps are presented is not limiting with respect to the order in whichthe steps may be executed.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A process for initializing a hydraulic hybrid vehicle system,comprising: signaling initialization of the system; checking a positionof a mode valve of the system; checking a displacement setting of ahydraulic motor of the system; coupling a high-pressure fluid supply tothe system if the mode valve is at a neutral position and thedisplacement setting is at zero, or if either the mode valve is at aneutral position or the displacement setting is at zero; and enablingoperation of the vehicle if, after performing the coupling step, themode valve is at the neutral position and the displacement setting is atzero.
 2. The process of claim 1, comprising: commanding the mode valveto a neutral position if, after performing the coupling step, the modevalve is not at a neutral position; rechecking the position of the modevalve after performing the commanding step; decoupling the high-pressurefluid supply from the system if, after performing the coupling step andthe rechecking step, the mode valve is not at the neutral position; andwherein the enabling step comprises enabling operation of the vehicleif, after performing the coupling step and the rechecking step, the modevalve is at the neutral position and the displacement setting is atzero.
 3. The process of claim 2, comprising providing a signal of acritical system fault to a driver, following the decoupling step.
 4. Theprocess of claim 1, comprising: commanding the displacement setting tozero if, after performing the coupling step, the displacement setting isnot at zero; rechecking the displacement setting of the hydraulic motorafter performing the commanding step; decoupling the high-pressure fluidfrom the system if, after performing the coupling step and therechecking step, the displacement setting is not at zero; and whereinthe enabling step comprises enabling operation of the vehicle if, afterperforming the coupling step and the rechecking step, the mode valve isat the neutral position and the displacement setting is at zero.
 5. Theprocess of claim 4, comprising providing a signal of a critical systemfault to a driver, following the decoupling step.
 6. The process ofclaim 1 wherein the coupling step comprises opening a valve between ahigh-pressure accumulator and the system.
 7. The process of claim 1,comprising: checking a position of a shift selector after the signalingstep and before performing the coupling step; and canceling the enablingstep if the shift selector is not in either a park or a neutralposition.
 8. The process of claim 7, comprising signaling a shiftselector position fault after performing the canceling step.
 9. Theprocess of claim 1, comprising: controlling, while a shift selector isin a position other than park or neutral, a displacement of thehydraulic motor, at least in part, in accordance with a position of anaccelerator pedal; and commanding the displacement of the motor to zerowhile the shift selector is in the park or neutral position.
 10. Theprocess of claim 9, comprising: starting, if an internal combustionengine is not already running, the internal combustion engine when theshift selector is moved to the park position; and running the internalcombustion engine while the shift selector is in the park position andoperation of the vehicle is enabled.
 11. A process for initializing ahydraulic hybrid vehicle system, comprising: signaling initializing ofthe system; coupling a low-pressure fluid supply to the system;comparing a flow of low-pressure fluid into the system with a firstthreshold value; enabling operation of the system if the flow oflow-pressure fluid does not exceed the first threshold value; anddecoupling the low-pressure fluid from the system if the flow oflow-pressure fluid exceeds the first threshold value.
 12. The process ofclaim 11, comprising providing a signal of a critical system fault to adriver, after performing the decoupling step.
 13. The process of claim11, comprising: comparing a flow of low-pressure fluid into the systemwith a second threshold value, lower than the first threshold value; andproviding a signal of a non-critical system fault to a driver if theflow of low-pressure fluid exceeds the second threshold value but notthe first threshold value.
 14. A process for initializing a hydraulichybrid vehicle system, comprising: signaling initializing of the system;coupling a high-pressure fluid supply to the system; comparing a flow ofhigh-pressure fluid into the system with a threshold value; enablingoperation of the system if the flow of high-pressure fluid does notexceed the threshold value; and decoupling the high-pressure fluidsupply from the system if the flow of high-pressure fluid exceeds thethreshold value.
 15. A method of operating a hydraulic hybrid vehiclesystem, comprising: coupling a high-pressure fluid source to the system;coupling a low-pressure fluid source with the system; measuring a fluidflow at the low-pressure fluid source; measuring a fluid flow at thehigh-pressure fluid source; comparing a reference value with an absolutevalue of a difference between the measured flow at the low-pressurefluid source and the measured flow at the high-pressure fluid source;and decoupling the high-pressure fluid source from the system if theabsolute value exceeds the reference value.
 16. The method of claim 15,comprising signaling a critical system fault after performing thedecoupling step.
 17. The method of claim 15, comprising: repeating themeasuring steps if the absolute value does not exceed the referencevalue; repeating the comparing step after performing the repeatedmeasuring steps; and decoupling the high-pressure fluid source from thesystem if, after performing the repeated comparing step, the absolutevalue exceeds the reference value.
 18. A method of operating a hydraulichybrid vehicle system, comprising: measuring a fluid flow at ahigh-pressure fluid source; calculating a fluid flow through a hydraulicmachine; comparing a reference value with an absolute value of adifference between the measured flow and the calculated flow; andrepeating the measuring, calculating, and comparing steps if theabsolute value does not exceed the reference value.
 19. The method ofclaim 18 wherein the measuring step comprises: obtaining a first fluidpressure value at a first point in a fluid transmission path; obtaininga second fluid pressure value at a second point in the fluidtransmission path; and deriving the measured fluid flow from the firstand second fluid pressure values and predetermined pressure dropcharacteristics of the fluid transmission path between the first andsecond points.
 20. The method of claim 18 wherein the calculating stepcomprises multiplying a displacement setting of the machine by an rpmsetting of the machine.
 21. The method of claim 18, comprising shuttingdown the system if the absolute value exceeds the reference value. 22.The method of claim 21 wherein the shutting down step comprises:commanding the hydraulic machine to a zero displacement; commanding amode valve to a neutral position; shutting down an internal combustionengine; closing a high-pressure fluid supply valve; and closing alow-pressure fluid supply valve after the commanding steps and after theshutting down step.
 23. A method of operating a hydraulic hybrid vehiclesystem, comprising: detecting displacement of a hydraulic motor;comparing detected displacement of the motor with a commandeddisplacement; repeating the detecting and comparing steps if an absolutevalue of a difference between the detected displacement and thecommanded displacement does not exceed a first reference value;signaling a non-critical system fault and repeating the detecting andcomparing steps if the absolute value of the difference between thedetected displacement and the commanded displacement exceeds the firstreference value but not a second reference value; and removing the motorfrom system operation if the absolute value of the difference betweenthe detected displacement and the commanded displacement exceeds thesecond reference value.
 24. The method of claim 23, comprising sending aplurality of high-pressure pulses to an actuator control valve.
 25. Themethod of claim 23, comprising commanding the motor to a displacement ofzero if the absolute value of the difference between the detecteddisplacement and the commanded displacement exceeds the second referencevalue, then repeating the detecting and comparing steps prior toperforming the removing step.
 26. The method of claim 25, comprising:deactivating a first actuator control valve if the absolute value of thedifference between the detected displacement and the commandeddisplacement exceeds the second reference value; activating a secondactuator control valve after performing the deactivating step;commanding the motor to a displacement of zero after performing theactivating step; repeating the detecting and comparing steps afterperforming the commanding step and prior to performing the removingstep.
 27. The method of claim 23 wherein the removing step comprisesshutting down the system.
 28. The method of claim 27, comprisingsignaling a critical system fault after performing the shutting downstep.
 29. The method of claim 23 wherein the removing step comprises:commanding a mode valve to a neutral position; confirming a position ofthe mode valve; signaling a non-critical system fault if, afterperforming the commanding step, the mode valve is in the neutralposition; and shutting down the system if, after performing thecommanding step, the mode valve is not in the neutral position.
 30. Themethod of claim 23 wherein the removing step comprises: closing anisolation valve; and signaling a non-critical system fault.
 31. Themethod of claim 23 wherein the detecting step comprises: obtaining afirst displacement value from a first displacement position sensor;obtaining a second displacement value from a second displacementposition sensor; comparing the first and second displacement values;providing the first displacement value as the detected displacement ifthe first and second displacement values are substantially equal; andproviding, as the detected displacement, the one of the first and seconddisplacement values that is closest in value to the commandeddisplacement if the first and second displacement values are notsubstantially equal.
 32. The method of claim 31 wherein the detectingstep comprises signaling a non-critical system fault after performingthe providing-the-one step.
 33. A method of operating a hydraulic hybridvehicle system, comprising: controlling, while a shift selector of thevehicle is in a position other than park or neutral, a displacement of ahydraulic motor, at least in part, in accordance with a position of anaccelerator pedal; and commanding the displacement of the motor to zerowhile the shift selector is in the park or neutral position.
 34. Themethod of claim 33, comprising: starting, if an internal combustionengine is not already running, the internal combustion engine when theshift selector is moved to the park position; and running the internalcombustion engine while the shift selector is in the park position andoperation of the vehicle is enabled.
 35. The method of claim 33,comprising: controlling, while the shift selector is in the parkposition, a throttle position of the internal combustion engine, atleast in part, in accordance with the position of the accelerator pedal;and controlling, while the shift selector is in a position other thanthe park or neutral positions, the throttle position of the internalcombustion engine in accordance with factors other than the position ofthe accelerator pedal.
 36. The method of claim 33, comprising: providinga signal if a driver of the vehicle leaves the vehicle driver's seatwhile operation of the vehicle is enabled.
 37. The method of claim 33,comprising: providing a signal if a driver of the vehicle leaves thevehicle driver's seat while the shift selector is in a position otherthan park.
 38. The method of claim 33, comprising: preventing removal ofa key from the vehicle while the shift selector is in a position otherthan park.
 39. the method of claim 33, comprising: setting adisplacement of the hydraulic motor to a value sufficient to cause thevehicle to creep in a direction indicated by the shift selector, whilethe shift selector is in a position other than park or neutral, anaccelerator pedal is not depressed, and the vehicle is traveling at lessthan a threshold speed; and setting the displacement of the hydraulicmotor to zero while the accelerator pedal is not depressed and thevehicle is traveling at more than the threshold speed.
 40. The method ofclaim 39 wherein the threshold speed is less than about five miles perhour.
 41. The method of claim 33, comprising: signaling a shut-down ofthe vehicle system; disabling operation of the vehicle; commanding thehydraulic motor to a zero displacement; commanding a mode valve to aneutral position; shutting down an internal combustion engine; closing ahigh-pressure fluid supply valve after performing the commanding stepsand the shutting down step; and closing a low-pressure fluid supplyvalve after performing the commanding steps.
 42. The method of claim 41,comprising: checking, immediately following the signaling step andbefore performing any other step, a position of a shift selector; andcanceling the remaining steps if the shift selector is not in the parkposition.
 43. The method of claim 41, comprising venting fluid pressurefrom a high-pressure portion of the system to a low-pressure portion ofthe system after performing closing the high-pressure fluid supplyvalve.
 44. The method of claim 33, comprising: controlling, while theshift selector is in a position other than park or neutral, a mode anddisplacement of the hydraulic motor, at least in part, in accordancewith a position of a brake pedal, such that when the brake pedal isapplied, a torque opposing a direction of travel of the vehicle isapplied; and reducing the displacement of the hydraulic motor toward adisplacement of zero as a speed of the vehicle approaches zero,regardless of the pressure applied to the brake pedal.
 45. The method ofclaim 44 wherein the reducing step comprises reducing the displacementof the hydraulic motor toward a displacement of zero as the speed of thevehicle approaches zero, such that the displacement of the motor is atzero while the vehicle is traveling below a threshold speed.
 46. Themethod of claim 33, comprising: momentarily reducing displacement of themotor, regardless of the position of the accelerator pedal or of a brakepedal if, while the vehicle is moving, a rotation speed of one of twodrive wheels is significantly greater than a rotation speed of the otherof the two drive wheels.
 47. A method of operating a hydraulic hybridvehicle system, comprising: measuring a fluid pressure within a casingof a hydraulic motor of the system; comparing the measured fluidpressure with a first reference value; closing a high-pressure fluidsupply valve if the measured fluid pressure exceeds the first referencevalue.
 48. The method of claim 47, comprising signaling a criticalsystem fault after performing the closing step.
 49. The method of claim47, comprising signaling a non-critical system fault if the measuredfluid pressure exceeds a second reference value but not the firstreference value.
 50. The method of claim 47, comprising disablingoperation of the system after performing the closing step.
 51. Themethod of claim 47, comprising moving an actuator control valve to aposition, if power to the actuator control valve is removed, such thatfluid pressure is applied to an actuator controlled by the actuatorcontrol valve to move a displacement of a hydraulic motor of the systemto a displacement of zero.
 52. The method of claim 51 wherein the movingstep includes applying a spring bias to a component of the actuatorcontrol valve.
 53. The method of claim 47, comprising: moving a modecontrol valve to a position, if power to the mode control valve isremoved, such that the motor is decoupled from a high-pressure fluidsupply.
 54. The method of claim 53 wherein the moving step includesplacing a first fluid port of the motor in fluid communication with asecond fluid port of the motor.
 55. A method of operating a hydraulichybrid vehicle system, comprising: measuring a fluid pressure of a firsthydraulic accumulator of the system; measuring a fluid pressure of asecond hydraulic accumulator of the system; predicting a fluid pressureof one of the first and second accumulators, based on the measuredpressure of the other of the first and second accumulators; comparingthe predicted pressure with the measured pressure of the one of thefirst and second accumulators; and shutting down the system if thepredicted pressure differs from the measured pressure by more than afirst threshold value.
 56. The method of claim 55, comprising signalinga critical system fault after performing the shutting down step.
 57. Themethod of claim 55, comprising signaling a non-critical system fault ifthe predicted pressure differs from the measured pressure by more than asecond threshold value and less than the first threshold value.